In motor vehicles, hospitals, etc., the CAN bus system has presently established itself as an affordable standard bus system. Messages or signals according to the CAN specification in ISO 11898 are transmitted therein between user stations which are also referred to as nodes. The user stations are, for example, control units or display devices of a motor vehicle, etc.
To allow for high data rates in the architectures of motor vehicle control units, for example, new high-rate bus systems are presently introduced. In this case, network sections are oftentimes built on the basis of different systems, LIN, CAN, FlexRay, Ethernet, etc., being used, for example. For this purpose, each bus system uses its own connection topology in the form of dedicated lines between the corresponding bus users.
It would be desirable, when a second bus system, which is in particular suitable for the high-rate transmission of data, could be designed in such a way that it may be operated on the same cable in parallel to an existing CAN bus, the first bus system. This requires an independent coexistence of the CAN bus system and the second bus system, i.e., of both systems.
An expansion of existing communication networks on the basis of existing standards to include further transmitting and receiving units by using separate channels is discussed in German Published Patent Application No. 103 01 637, for example.
Studies have shown that designing two systems on different frequency ranges is by itself not sufficient for a coexistence as long as the CAN bus, as one of the two systems, couples into the bus dominant and recessive bit states having different impedances with the aid of customary transmitting and receiving units, which are also referred to as transceivers. A technology of this type is, however, the basis of the CAN transmission mechanism including the arbitration. Changing this would intervene too strongly into the system properties of the CAN transmission mechanism.
It is problematic that a change in the bus impedance in accordance with the starting condition of the CAN transceiver causes a transmission medium which is no longer statically acceptable. A change may be modeled as a node impedance of the CAN nodes which changes over time. Moreover, these impedances are in general a function of the frequency.
In addition to this impedance effect, an interference for the second bus system in the form of harmonic waves of the CAN transmission exists on the bus line. The performance density of the harmonic waves, however, strongly drops toward higher frequencies. The interference may be limited to a tolerable degree with the aid of a suitable selection of the, in particular high-frequency, spectrum and design of the transmitting power of the second bus system. This, however, results in the problem that the non-static bus impedance manifests itself in a multiplication of the, in particular high-frequency, signal of the second bus system by the CAN signal and results in additional components in the received signal. The power of these portions scales with the transmitting power of the second bus system and therefore results in an inherent interference power which increases proportionally to the transmitting power. This may result in that a reception is no longer possible under the customary quality requirements, in particular with regard to the error rates which are to be complied with.
For these reasons, an interconnection of the CAN bus system with a second bus system is usable only to a limited extent without further measures.